This invention relates to heat recuperators, and more particularly relates to a ceramic heat recuperative structure and recuperator assembly employing a ceramic core and a metal housing, for use on furnaces, calciners, ovens and preheaters.
Recent concern about energy conservation and rising fuel costs has caused renewed interest in industrial recuperators to recover waste heat losses and preheat incoming combustion air and/or fuel to increase the efficiency of furnaces, calciners, ovens and preheaters.
While such recuperators are usually constructed from metal parts, the ceramic recuperator has several advantages over conventional metallic recuperators. For example, ceramics in general have high corrosion resistance, high mechanical strength at elevated temperatures, low thermal expansion coefficients (TEC's) and good thermal shock resistance, and thus exhibit excellent endurance under thermal cycling; are light in weight (about one third the weight of stainless steel); and are cost competitive with high temperature alloys.
Furthermore, ceramic recuperators are available in a variety of shapes, sizes, hydraulic diameters, (hydraulic diameter is a measure of cross-sectional area divided by wetted parimeter) and compositions. Because their TEC's are typically lower than those of most metals and alloys, however, ceramic recuperators present a compatibility problem to the design engineer desiring to incorporate them into housings for retrofitting on furnaces, calciners, ovens and preheaters.
In U.S. Pat. No. 4,083,400, issued Apr. 11, 1978 and assigned to the present assignee, a ceramic cross-flow recuperator core is incorporated into a metallic housing adapted for retrofitting to the metallic fittings of existing furnaces, calciners, ovens and preheaters. Insulating and resilient sealing means between the core and housing minimize heat loss through the metallic housing and prevent leakage of heat transfer fluids, such as exhaust flue gases and incoming combustion air, past the core.
An additional problem in ceramic recuperator design is leakage of the heat transfer fluids between layers of the ceramic core structure itself, resulting in decreased overall efficiency of the recuperator apparatus. Interlayer leakage is particularly troublesome in forced-draft applications where a significant backpressure is built up on the imcoming air side of the recuperator. Such cool incoming air leaks into the hot flue or exhaust gas, reducing the heat content of that gas which would otherwise be available to heat the incoming air. In one design, alternate stacked ribbed layers form the cross-flow paths for the heat transfer fluids. While firing of stacked layers results in bonding together by sintering at points or areas of contact between the green layers, resulting in a unitary structure having mechanical strength, nevertheless deviations from planarity of the stacked green layers results in incomplete sintering together of these layers, leaving voids or cracks along the contact surfaces. Some of these voids or cracks may be evident at the visible edges or "bond lines" of the bonding surface between the outermost rib of one layer and the flat surface of the base portion of an adjacent layer.